1. Field of the Invention
This invention relates to a method for improving the printability of plastic surfaces and, more particularly, to a method for improving the printability of surfaces of polyolefin resin moldings by exposing the surfaces to ultraviolet radiation. Still more particularly, it relates to a method for improving the printability of a surface of the barrel of a syringe (hereinafter referred to as "syringe barrel") formed of a polypropylene resin by exposing the surface to ultraviolet radiation.
The surface treatment method of the present invention is characterized in that the printability of surfaces of polyolefin resin moldings is improved by exposing the surfaces to a specific kind of ultraviolet radiation.
2. Description of the Prior Art
It is generally known that, if it is desired to print surfaces of polyolefin resin moldings, the printability of such surfaces must be improved by a suitable pretreatment (i.e., surface treatment) because polyolefin resins are inert to printing inks.
Especially where an ultraviolet radiation-curing ink comprising an acrylic resin as the base material is used, there is a limit in improving its affinity for polyolefin resins. Moreover, since the ink gives a cross-linked and cured film, a separation tends to occur at the interface between the resin surface and the ink film. Thus, a more adequate pretreatment is required in this case.
Conventionally, a number of surface treatment methods have been proposed with a view to improving the printability of surfaces of polyolefin resin moldings, and some of them have been put to practical use.
These surface treatment methods include, for example, plasma treatment in which a low-temperature plasma is generated under reduced pressure and brought into contact with a surface to be printed of a molded article; corona discharge treatment in which a plasma is generated by a corona discharge at atmospheric pressure and brought into contact with a surface to be printed of a molded article; and flame treatment in which a gas is burned and the plasma generated in the flame is brought into contact with a surface to be printed of a molded article. Although plasma treatment produces a satisfactory surface-treating effect, it requires operation under reduced pressure and must be carried out in a batchwise manner, except under special circumstances. In many cases, therefore, the manufacturing process is difficult to rationalize and the processing cost is high. In corona discharge treatment, sheet materials can be continuously processed at atmospheric pressure and the processing cost is low enough. However, non-sheet materials may not be processed at all or, if possible, special processing equipment will be required. Moreover, it is known that the effect of the surface treatment will diminish with time in the case of polyolefin resins and, in particular, polyethylene resins. That is, the printability of the surface-treated articles may deteriorate after long-term storage. For molded articles unfit for the foregoing methods (e.g. relatively large-sized articles having undulating surfaces), flame treatment is often employed. However, this method has the disadvantage that the effect of the surface treatment varies considerably. Moreover, this method is often unapplicable to molded articles having certain shapes, because it is impossible to prevent them from being deformed by heat.
For epoxy resins, there has recently been proposed a method for surface-treating molded articles by use of a lamp capable of efficiently emitting ultraviolet radiation having wavelengths of 185 nm and 254 nm. In other words, this method comprises exposing surfaces of such molded articles to ultraviolet radiation from a low-pressure mercury vapor lamp. This method is advantageous in that it is applicable to molded articles having a wide range of shapes and such molded articles can be processed in a rationalized production line. For polyolefin resins, however, it is less effective to expose their surfaces to ultraviolet radiation from a conventional low-pressure mercury vapor lamp for a short period of time. Accordingly, this method cannot be employed in an actual production line without modifying the primary structure of the resin or adding a suitable additive thereto. This has led to the disadvantage that the resin most suitable for the intended purpose of the product may not be chosen because of its lack of printability.
The molded articles to which the present inventors' desire for the improvement of printability is especially directed are medical containers formed by blow molding of polyolefin resins in which the wall constituting the surface to be printed or areas around it has a thickness of 1 mm or less. The conventional methods could never been applied to such molded articles because severe restrictions are imposed on their material (owing to their medical use), shape (owing to their use as containers), manufacturing process, processing cost and the like.
Also in the case of syringe barrels having a shape which can be more easily surface-treated than the aforesaid medical containers, similar restrictions are imposed thereon, though not so severe as for the aforesaid medical containers. Consequently, it is the existing state of the art that untreated surfaces of syringe barrels formed of polypropylene resins are printed with an oil ink having an affinity for the resins. For this reason, most of the ink films on commercially available syringes formed of polypropylene resins are removed simply by rubbing them with a hand.
Moreover, syringes are used for a variety of medical fluids, some of which have the nature of an oil or oil-like substance. If such a medical fluid comes into contact with the surface of a syringe barrel, the ink film will be dissolved away and, in the worst case, the graduations cannot be read at all. That is, a solvent type ink having an affinity for polypropylene resins also has an affinity for oils, so that the ink film will be dissolved thereby.